1. Field of the Invention
The present invention relates to an apparatus for ejecting droplets capable of ejecting minute droplets, an actuator controller used in the apparatus, and a method for controlling the actuator.
2. Description of Related Art
A printing head in an inkjet printer includes therein an ink tank and pressure chambers. Each pressure chamber is supplied with ink from the ink tank. An actuator is arranged to correspond to each pressure chamber. When the actuator is driven to change a volume of a pressure chamber, ink in the pressure chamber is pressurized to be ejected through a nozzle connected with the pressure chamber. Particularly in order to eject minute droplets for high-resolution printing, there is known a technique, as disclosed in U.S. Pat. No. 6,350,003, in which an ejection pulse signal and an additional pulse signal accompanying the ejection pulse signal are sequentially applied to an actuator in accordance with a one-dot printing instruction.
According to the above technique, an application of the ejection pulse signal to the actuator increases the volume of a pressure chamber and ink is supplied to the pressure chamber from an ink tank. Then, when the application of the ejection pulse signal to the actuator completes, the volume of the pressure chamber decreases and ink protrudes from a nozzle. When the ink protruding from the nozzle is still connected with the nozzle and an ink droplet is going to be ejected, the additional pulse signal is applied to the actuator. Then, the volume of the pressure chamber again increases and a tail portion of the ink protruding from the nozzle is pulled back toward the pressure chamber, so that a part of the ink protruding from the nozzle other than the tail portion becomes apart from the nozzle to be ejected as an ink droplet. Like this, since the tail portion of the ink protruding from the nozzle to be ejected is pulled back toward the pressure chamber, an actually ejected ink droplet can be small in volume. Given that a time required for a pressure wave to propagate in one way longitudinally through the pressure chamber is T, a pulse width of the ejection pulse signal is set to be T, a pulse width of the additional pulse signal to be 0.2 T to 0.6 T, and a time interval between a completion of an application of the ejection pulse signal and a start of an application of the additional pulse signal to be 0.3 T to 0.7 T.
However, when each pulse signal is applied in accordance with the pulse width and the application timing as described above, an ejection speed of a droplet becomes relatively low. A low ejection speed of a droplet causes an unstable ejection to thereby deteriorate printing quality.